This invention pertains generally to ventilation systems designed for use with electronic equipment, such as personal computers, servers, etc., and more particularly, to a novel electro-magnetically shielded ventilator grill for use in connection with forced air ventilation systems.
As the operational speeds of computers and computer circuits have increased over the years, so has the emitted electro-magnetic (EM) radiation of components used in such computers and circuits. Increased levels of EM radiation are to be expected with higher frequency signals. At frequencies below a few 100 kHz component leads of a few millimeters act as simple conductors, yet at frequencies above 50 MHz the same conductors can become antennas radiating EM energy in all directions. EM radiation is an inherent problem in designing high speed digital circuits. Stray radiation causes electro-magnetic interference (EMI) in surrounding electronic systems which often causes abnormalities to occur in those systems. Accordingly, numerous EM emission standards have been established by governmental regulatory agencies setting limits on the amount of radiated and conducted energy from electronic systems. Such standards vary depending upon the type of electronic system, the environment of use and various other factors. (For example, the subject apparatus described herein is designed to meet the emission standards of the 1993 FCC class B requirements for unintentional radiators, Title 47 part 15, and CISPR 22, level B requirements.)
It is well known that if the source of EM radiation cannot be eliminated or removed from close proximity to other electronic parts that may act as receptors, the only sure way to prevent the unwanted electro-magnetic interference is by shielding the radiator, the receptor or both. Basically EM shielding suppresses the radiated energy by covering the radiator/receptor with an electrically conductive material. In the case of a computer system, the shielding takes the form of a closed metallic cabinet (a chassis) which houses and provides mechanical support for the computer circuit boards.
Forced air ventilators are particularly a problem for shielded enclosures, since fan grill openings offer unusually large leakage paths to radio frequency signals. Eliminating the fan grill and the entire ventilation system is one solution to containing emissions, albeit impractical, but nearly every computer system from a common personal computer (pc) to a large mainframe computer requires some form of ventilation system to exhaust excess heat. The traditional wire finger guard covering a fan blade aperture provides ample air flow but virtually no resistance to EM signals. Conversely, simple circular hole patterns punched into a chassis (see FIG. 1 ) have been used with success to limit radiation, but the volume of air flow is not optimum, and the small apertures create enough back pressure to require a plenum interfacing the grill and the fan blades to reduce air turbulence at the interface of the grill and the fan blades. In some cases the air flow is so poor that multiple ventilator systems have been required (see, for example, U.S. Pat. No. 4,751,872).
Accordingly, this invention is directed to an inexpensive EM shielded fan grill design particularly useful when used in combination with metallic cabinets for electronic equipment.
A principal object of this invention is the provision of a simple fan grill that provides EM shielding and increased air flow over previous ventilator designs.